Add a basic tox.ini file (fixes #6)

I've added a basic tox.ini which:
* runs the tests using trial under python2.7
* prints a coverage summary
* prints a pyflakes report

Also added a few paragraphs to README.rst explaining how to run the tests.

The tests are currently `trial` specific, otherwise I'd have used py.test
And since `trial` hasn't been ported to python3, I can't attempt to run the tests under python3.4. 
See #16 for removing twisted dependencies.

Once this is merged I can create a travis recipe to run the tests and pyflakes, automatically via tox. See #5 

And  once the tests are running via travis, I can get coveralls running. See #15.

Fixes #6 
I'd copied and pasted those from the cryptography project. Now fixed.